The advent of portable video cameras that allow operators to see plumes from gas leaks promises to improve air quality while saving industry millions of dollars in compliance expenses and lost products. Small, economical, and accurate, these devices could be commercially available in two or three years. Scientists and engineers at Sandia National Laboratories are pushing development by employing some of the very newest optical materials and laser technology to whittle down the size and scale up the power of these plume-imaging devices. Their goal is to build portable leak detectors capable of detecting fugitive emissions of volatile organic compounds (VOCs) at mass leak rates of < 2 grams per hour from distances up to 40 meters.
Stopping Leaks Is Critical, But Finding Them Is Expensive
Fugitive hydrocarbon emissions contribute to greenhouse-gas generation and tropospheric ozone and smog. The U.S. Environmental Protection Agency (EPA) estimates that methane, much of it from leaks, contributes up to 15 percent of total greenhouse gas emissions. Thus, detecting and stopping leaks is a major international concern. Modern chemical plants, oil refineries and natural-gas distribution systems move millions of tons of feedstock and products through mazes of pipes and valves. A single refinery can have over 40,000 valves. Plumbing of this size and complexity affords thousands of places for potential leaks.
Fugitive hydrocarbon emissions contribute to greenhouse-gas generation and tropospheric ozone and smog.
Finding leaks costs lots of money. For example, the natural-gas industry alone spends hundreds of millions of dollars each year on locating leaks in the 1.8 million miles of its gas-delivery system. Refineries have seen their monitoring costs rise as the number of monitoring points increases: in one case from 100,000 to 300,000.
Valuable product is also lost. Although less than one percent of their gas is lost to leaks, the natural gas industry reckons it loses 1014 British thermal units (Btu) per year, fuel worth $200 million, which could heat 1.2 million homes.
Laws covering gas leaks have been around since the late 1970s. Leak detection and repair (LDAR) programs to reduce emissions of VOCs and other hazardous air pollutants were first instituted by the United States. On the national level, the Clean Air Act Amendments of 1990 set allowable leak rates and monitoring frequencies for each industry segment. Today, virtually every refinery in the country is required under National Emission Standards for Hazardous Air Pollutants (NESHAPs) and New Source Performance Standards (NSPS) rules to have an LDAR program in place. Both NSPS and NESHAPs require testing of individual components in accordance with EPA Method 21, which stipulates among other things that conventional point gas detectors be placed as close as one millimeter (mm) to the component being monitored. Using Method 21 to track down fugitive emisions can be very time consuming and expensive because of the large number of components in a modern refinery or chemical plant (remember those 40, 000 valves). Furt
hermore, the guidelines for fugitive gas emissions have become increasingly stringent since 1990.
The natural-gas industry alone spends hundreds of millions of dollars each year on locating leaks in the 1.8 million miles of its gas-delivery system.
New Technology Promises Faster and Cheaper Detection
Both industry and the EPA have come to realize that the regulatory guidelines have outstripped the Method 21 technology. Fortunately, more technologically advanced solutions are now available. Among the competing technologies, backscatter absorption gas imaging (BAGI) has the advantage of allowing simultaneous viewing of many potential leak sites over an extended area and pinpointing a leak by direct visual observation. Also, because the entire leak can be seen with BAGI, the magnitude of the leak can usually be estimated. BAGI relies on infrared laser radiation to illuminate an area and the backscattered radiation to produce a real-time video image. Thus, if a gas plume is present in the imaged zone, and some of the backscattered laser light is attenuated, the plume appears as a dark cloud on the video picture. Sample video of plume images collected during calibration tests at Sandia National Laboratory can be viewed on the Sandia Combustion Research Facility Web site (www.ca.sandia.gov/CRF/).
Engineered Crystals and Compact Amplifiers Mean Better Imagers
Sandia's contribution to gas imaging has been to develop suitably compact, powerful and tunable laser light sources. These have led to a completely portable, lightweight (20 lbs.), shoulder-borne, gas-imaging camera. The Sandia remote imaging team has been able to take advantage of two relatively new technologies: quasi-phasematched (QPM) nonlinear materials and fiber amplifiers. Early BAGI imagers were based on continuous wave (cw) CO2 lasers that operated in the 9-11 micrometer spectral range, a region inadequate for hydrocarbon detection. These devices are currently being produced by Laser Imaging Systems in Punta Gorda, Fla.
In contrast, the tunable new nonlinear crystals operate between 3.1-3.7 micrometers, a region well suited to detecting methane and the aliphatic hydrocarbons most likely to be present at refineries and chemical plants. The engineered crystals also have much higher gain, which allows for small sensors and the use of lower-power pump sources, such as diodes or fiber amplifiers. They are also broadly tunable, which enables the detection of many chemicals with one laser. On the horizon, even newer engineered crystals could push operation into the long-wave infrared region, making it possible to detect more distinct functional groups.
Fiber laser amplifiers are the other key to building field-portable gas detectors. A van-mounted imager tested in 1999 employed a Nd:YAG pump laser, which required a water chiller for cooling and a gasoline-powered generator for electrical power. In the new portable version, an air-cooled, fiber laser amplifier and a backpack-borne battery have replaced the water-cooled imager and the generator. Widely used in the telecommunications industry, fiber-amplified lasers are compact, lightweight, rugged and electrically efficient. Moreover, the single-mode fiber core ensures intrinsically high beam quality that is insensitive to mechanical vibrations or optical power level.
Backscatter absorption gas imaging (BAGI) has the advantage of allowing simultaneous viewing of many potential leak sites over an extended area and pinpointing a leak by direct visual observation.
The Sandia team is truly excited about their VOC imager. For those who have spent their technical careers in the arcane world of laser research, the prospect of building hardware that might be widely used in the real world is thrilling. The team field-tested a van mounted gas-imaging instrument at a Gulf Coast petroleum refinery during 1999. This imager identified large and mid-size hydrocarbon leaks, some of which were not on the ordinary leak survey schedule. Overall, the results compared favorably with parallel measurements made by Method 21. The much smaller operator-portable version is now bound for April field tests being conducted together with the American Petroleum Institute (API) and the U.S. EPA at a Texas refinery.
This article originally appeared in the May 2001 issue of Environmental Protection, Vol. 12, No. 5, p. 32.
This article originally appeared in the 05/01/2001 issue of Environmental Protection.